The nation's most populous state discards more than 40 million tires every year. In addition to managing the waste, public works departments work with the California Integrated Waste Management Board to reuse the waste.

Cities and counties can get grants of up to $500,000 a year to use rubberized asphalt concrete in hot mixes and chip seals. But if they can use tire-derived aggregate for another application, the board gives it away and charges only hauling costs. That's one reason the Sonoma County Department of Transportation and Public Works uses it to prevent landslides along some of its 1,300 miles of road.

"The use of rubber aggregate in the design of the repair doubles the safety factor due to the light weight," says Deputy Director Tom O'Kane. "The material is easy to spread, compact, and cover. As long as there's a supply of chipped tires, we'll use it in these cases."

He tried it out less than a year ago to repair a 75-foot slide along 1,000 feet of rural road. The aggregate ranges in size from 25 millimeters to 300 millimeters and costs $25 to $55 per ton. It's highly permeable and absorbs vibrations well, making it an attractive alternative to building retaining walls or importing volcanic rock from other areas of the state. The department recently saved $300,000 on another project, for which the board delivered 6,400 cubic yards.

Nationwide, 60 million tires are converted into aggregate each year, and 13 states allow it as a lightweight fill for highway embankments.